In recent years, the so-called RF plasma CVD process has been frequently used for the production of semiconductor devices. In the RF plasma CVD process, a high frequency of 13.56 MHz is generally used in view of the wireless telegraphy act. The RF plasma CVD process has advantages in that the discharge conditions can be relatively easily controlled and the quality of a film obtained is excellent, but it has drawbacks in that the utilization efficiency of a film-forming raw material gas is not sufficient enough and the deposition rate of a film is relatively small. In order to solve these drawbacks in the RF plasma CVD process, there has proposed a microwave plasma CVD process using a microwave with a frequency of 2.45 GHz. The microwave plasma CVD process has a pronounced advantage which cannot be attained by the RF plasma CVD process. That is, according to the microwave plasma CVD process, there can be attained the formation of a deposited film at an extremely high gas utilization efficiency and at a markedly high deposition rate. Japanese Laid-open patent application No. 186849/1985 (hereinafter referred to as Document 1) discloses an example of such microwave plasma CVD process. Particularly, in Document 1, there is described a microwave plasma CVD process using a microwave plasma CVD apparatus of the constitution shown in FIG. 1.
In the following, description will be made of the microwave plasma CVD technique described in Document 1.
In FIG. 1, reference numeral 2222 indicates a vacuum chamber (a deposition chamber). In the vacuum chamber, a plurality of rotary shafts 2238 are arranged in substantially parallel to each other. On each of the rotary shafts, a cylindrical substrate 2212 is positioned such that it can be rotated. Each cylindrical substrate 2212 can be rotated by virtue of a driving force transmitted from a motor 2250 through a driving chain 2264. In FIG. 1, there are shown only two cylindrical substrates for the simplification purpose. Actually, six cylindrical substrates 2212 are concentrically arranged such that they are spacedly positioned while maintaining a desired space between each adjacent cylindrical substrates. Reference numeral 2232 indicates an inner chamber (that is, a discharge space) which is circumscribed and formed by the six cylindrical substrates 2212. Reference numeral 2268 indicates a plasma generated in the inner chamber 2232. Reference numeral 2294 indicates a microwave transmissive window positioned at one end side of the inner chamber 2232. The microwave transmissive window is connected to a microwave power source (a magnetron) 2270 through waveguides 2282 and 2278. Reference numeral 2274 indicates an antenna probe which is extending into the waveguide 2278 from the microwave power source 2270. Reference numeral 2296 indicates another microwave transmissive window disposed at the remaining end side of the inner chamber 2232. This microwave transmissive window is connected to a a microwave power source (a magnetron) 2272 through waveguides 2284 and 2280. Reference numeral 2276 indicates an antenna probe which is extending into the waveguide 2280 from the microwave power source 2272. Microwave energy from each of the microwave power sources 2270 and 2272 is transmitted to the waveguides (2278 and 2282, or 2280 and 2284) through the antenna probe 2274 or 2276, and it is introduced into the inner chamber 2232 through the microwave transmissive window 2294 or 2296.
Film formation in the microwave plasma CVD apparatus shown in FIG. 1 is conducted as follows. That is, The vacuum chamber 2222 is evacuated through an exhaust port 2224 to bring the inside thereof to a desired vacuum. Thereafter, raw material gas is introduced into the inner chamber 2232 through gas feed pipes 2226 and 2228. Then, microwave energy is supplied into the inner chamber 2232 from the upper and lower sides of the inner chamber, wherein the raw material gas is decomposed by the action of the microwave energy to generate plasma 2268, resulting in forming a deposited film on the surface of each of the cylindrical substrates 2212 maintained at a desired temperature by means of electric heaters 2200.
Document 1 describes that according to the microwave plasma CVD apparatus shown in FIG. 1, a deposited film can be formed on each of the cylindrical substrates 2212 at a high deposition rate and at a high gas utilization efficiency. In the case of the microwave plasma CVD apparatus shown in FIG. 1, since microwave energy is used as above described, the density of the plasma generated upon the film formation becomes extremely high and because of this, the decomposition of raw material gas rapidly proceeds to cause the formation of a deposited film at an increased deposition rate. Thus, there is a problem in that it is extremely difficult to stably form a deposited film having a tense texture. In addition to this problem, there are also another problems. That is, since microwave energy is supplied into the inner chamber 2232 through the microwave transmissive windows 2294 and 2296 to cause the decomposition of raw material gas in the inner chamber, film deposition unavoidably occurs also on the microwave transmissive windows 2294 and 2296, wherein films thus deposited on these microwave transmissive windows prevent the microwave energy from efficiently transmitting through the microwave transmissive windows and in addition to this, the films deposited on the microwave transmissive windows are liable to peel off and contaminate into films deposited on the cylindrical substrates. Therefore, it is essential to periodically conduct a work of removing the films deposited on the microwave transmissive windows.
Document 1 discloses, other than the aforesaid microwave plasma CVD apparatus, a plasma CVD apparatus using a radio frequency energy (a RF energy). This plasma CVD apparatus is of the constitution shown in FIG. 2. The apparatus shown in FIG. 2 is a partial modification of the microwave plasma CVD apparatus shown in FIG. 1 in which the microwave energy introducing means is replaced by a RF energy introducing means comprising an antenna 2236. Particularly, the apparatus of FIG. 2 is constituted such that the two microwave energy introducing means of the apparatus of FIG. 1, each comprising the microwave power source, waveguides and microwave transmissive window, are removed, the installation position for one of said two microwave introducing means is sealed by a plate 2232, and an antenna is disposed at the installation position for the remaining microwave introducing means such that it extends into the inner chamber 2232. Reference numeral 2434 indicates a plate which serves to seal an opening of the upringht-standing wall 2334 which has been caused as a result of having removed the waveguide 2282. The antenna 2236 is supported by an insulating plate 2238, and it is electrically connected to a radio frequency power source (not shown) through a lead wire 2340. The antenna 2236 and the plate 2434 are designed to form a coupling means for introducing a radio frequency energy into the inner chamber 2232.
Document 1 describes that according to the apparatus shown in FIG. 2, plasma 2268 can be formed in the inner chamber 2232 by using a radio frequency energy. However, the apparatus shown in FIG. 2 has problems in that since the coupling means is constituted by the antenna 2236 and the plate 2434 and the supply of the radio frequency energy into the inner chamber 2232 is performed chiefly through the tip portion of the antenna 2336, the plasma generated in the inner chamber is liable to become uneven in the direction along the longitudinal axes of the cylindrical substrates 2212 and because of this, it is extremely difficult to form a homogeneous deposited film having a uniform thickness on each of the cylindrical substrates. This situation can be easily understood with reference to the results obtained in the experiments by the present inventors as for the process described in Document 1, which will be later described. Further, the use of a radio frequency energy is described in Document 1, but the document does not detail anything about the frequency thereof.
By the way, in recent years, studies have been made of a plasma CVD process using a very-high-frequency of the so-called VHF range, having a frequency of 30 MHz to 150 MHz which is greater than the 13.56 MHz but smaller than the microwave. For instance, in Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, pp. 267-273 (1987) (hereinafter referred to as Document 2), there is described a film-forming manner using a glow discharge decomposition apparatus of the capacitively coupled type wherein raw material gas (silane gas) is decomposed by using a very-high-frequency energy of 25 MHz to 150 MHz to form an amorphous silicon (a-Si) film. Particularly, Document 2 describes that a-Si films are formed with a different frequency in the range of 25 MHz to 150 MHz, in the case of using a frequency of 70 MHz, the film deposition rate becomes 21 .ANG./sec. which is the highest, this film deposition rate is 5 to 8 times that in the RF plasma CVD process, and the defect density, optical band gap and conductivity of the resulting a-Si film is slightly influenced by the excitation frequency employed. However, Document 2 is of the film formation at a laboratory scale but it does not mention anything of whether or not the foregoing effects are provided also in the formation of a large area film. Further, Document 2 does not mention anything about a manner of efficiently forming a large area film concurrently on a plurality of substrates to produce a plurality of practically usable large area semiconductor devices. In fact, Document 2 merely suggests a possibility in the future by saying that the use of higher frequencies (13.56 MHz to 200 MHz) opens interesting perspectives for fast processing of low cost, large area a-Si:H film devices in which thicknesses of several um are required.
In addition, Japanese Laid-open patent application No. 64466/1991 (hereinafter referred to as Document 3) discloses a manner of forming an amorphous silicon series semiconductor film on a cylindrical substrate by using a very-high-frequency energy of greater than 20 MHz (preferably, 30 MHz to 50 MHz). Particularly, there is described a manner in which raw material gas is introduced into a reaction chamber, the inside of the reaction chamber is maintained at a gas pressure of 10.sup.-4 to 0.2 Torr, and a very-high-frequency energy in a quantity corresponding to 0.1 to 10 W/sccm in terms of a ratio to the flow rate of the raw material gas is introduced into the reaction chamber to cause glow discharge whereby forming said amorphous silicon series semiconductor film. Document 3 describes that according to this manner, a film deposition rate of more than 10 um/hour can be attained and the resulting deposited films can be made to be of less than 20% in thickness variation.
However in the case of the manner of Document 3, when the above film deposition rate is tried to attain by using a very-high-frequency having a frequency beyond the above mentioned frequency range, there cannot be obtained a satisfactory result. Further, Document 3 does not mention anything about a manner of efficiently forming a large area film concurrently on a plurality of substrates to produce a plurality of practically usable large area semiconductor devices.